The present invention relates, in general, to tunable laser spectroscopy, and more particularly to laser spectroscopy utilizing an optical parametric oscillator as a continuously tunable laser source.
For many spectroscopic and spectrometric applications, such as nonlinear, photothermal, and fluorescence spectrometry, widely and continuously tunable laser sources are required. Until recently, dye lasers have been generally used for tunable laser spectroscopy. However, the tuning range of dye lasers tends to be severely limited. Each dye can cover only a few hundred angstroms, and the total range that can be covered by laser dyes is limited from approximately 400 nm to 1 .mu.m. To extend beyond the primary dye laser tuning range, complicated nonlinear optical techniques are required.
Thermal lens spectrometry is known as a highly sensitive method for detecting very small quantities of material by absorption of visible, ultraviolet or infrared light from a laser source. This method takes advantage of the thermal lens effect, which is a thermally induced alteration of the index of refraction of an absorbing medium which occurs when a laser beam is passed through the medium. In the thermal lens device described in U.S. Pat. No. 4,310,762, a converging beam derived from a coherent, collimated beam is passed through a reference cell. The converging beam is slightly modified by the change in index within the cell, due to the thermal lens effect. The modified beam then is passed through a sample cell containing the identical medium, with an additional medium which is to be identified. The reference cell and the sample cell are located at points in the beam path so that any modification in the beam caused by a change in the index of refraction in the reference cell is canceled by the same medium in the sample cell. Any detectable modification in the beam, e.g. expansion or divergence, as it evolves from the sample cell is due to the thermal lens effect caused by an additional pump beam directed onto the material to be identified.
U.S. Pat. No. 4,544,274 also uses the thermal lens phenomenon for measuring weak optical absorptions when a cell containing sample is inserted into a normally continuous-wave laser-pumped dye laser cavity. The pulsewidth of the resulting pulsed laser output is related to the sample absorbtivity by a simple calibration curve.